Hi @pipcart,

Your ASHP is barely ticking over and is almost certainly starting and stopping a lot.  It has a baseline power consumption just to keep it running and it'll be delivering very little energy to keep the rads at 24 degrees.  In addition, the Legionnaire's cycle will skew the numbers more than when running normally.  I'd expect power consumption and COP to be low in these circumstances. 

In addition, the internal estimates of energy consumption and delivery are often very inaccurate at times of low use. 

I wouldn't worry about it unless consumption seems excessive.  Wait until you're using it in anger to see how it's performing.  It shouldn't spend long periods iced up though.  It will ice up in cold weather but should spend most of its time running normally.  Did the installers have anything to say about that? 

Thanks Kev M, that's reassuring, although the COP curves I have found online show increasingly high efficiency as radiator temperatures drop, even into the low 20s C. Also, the temperature curve drops to 20 in warmer weather, so I'm not clear how the pump will perform then.

I think you are right that we will not really get a clear picture until we are living in the house.

The installers were concerned about the icing, which improved once weather dependency was switched on.

At present, the cost of running the unit is outweighing the benefits. I could reduce cost by returning to a fixed temperature, but that seems a bit daft.

@pipcart 

When the ASHP COP is tested it's not done in a real house but on some sort of test rig. On the test rig, even though the flow is only 20 degrees, the ASHP will be running continuously at its nominal capacity, which means it will be outputting 12kW or so. That won't ever happen in a real house; like I said yours is switching on and off and outputting a fraction of that. It's like mpg figures; comparing stop start city driving with driving along a long, flat, smooth road at the same average speed.  

Running at a higher flow at very low demands isn't such a daft idea.  This can lead to less cycling (ons and offs) and may be a better way to run your system where the heat demand is very low. I don't know how your Daikin is set up but can you run on weather compensation and use a thermostats that can stop the ASHP when it gets to a certain room temp?

Although it goes against all normal ASHP advice, in your situation you might even be better off heating your house to a higher temperature for part of the day rather than keeping it very low all the time.   

@Kev M

Thanks for the further advice. I'm going to turn the offset temp (flow) up and use the room thermostat to limit the room temperature, which is what I think you are suggesting.

I'm now getting 1/2 hourly data from our smart meter and temperature lovers inside and outside the house, so I should be able to evaluate the effect of the change.

I'll post the results here.

Using our house as an example, with a system that is by no means as efficient as it should be, our COP for the past fortnight has been 2.5 (running on weather compensation) with ambient temperatures of 0-8C, so I would have expected your system to certainly be over 1 even with the low target temperatures. Frequent stop/starts as Kev has pointed might be the issue here.

What is your electricity consumption like?